Italwars,
I'm in the USA not UK, but I am a long-time Second Afghan War/NWF miniature gamer and for better or worse I probably spend more of my time reading and researching than actually gaming, so I will venture to add my two cents/pence...
YES, go ahead and raise full units of PIONEERS and/or SAPPERS & MINERS. There were some differences between the two troop types, but they could accomplish many of the same tasks.
Pioneers were raised in Regiments of at least one battalion and those battalions were often assigned as part of a larger force (Brigade, "Field Force"/Division) and fought in various battles, such as the 23rd Bengal Native Infantry (Pioneers), later known as the 23rd Sikh Pioneers, which fought at Peiwar Kotal, Charasiab, and Kabul during the Second Afghan War. Pioneers specialised in clearing obstacles, building bridges, building or improving roads, etc.
Sappers & Miners were raised in Company size units, such as No. 7 Company Bengal Sappers & Miners, which fought at Charasiab and Kabul (each of the three Presidency Armies -- Bengal, Madras, and Bombay -- maintained Sapper & Miner companies). They specialised in the same sort of construction and destruction as Pioneer regiments but with added expertise in the use of explosives for clearing obstacles, breaching fortifications and destroying buildings. In reality I beleive these distinctions in the units' ablities related more to their predecessors from the 18th and earlier 19th Centuries, and by the time of the Second Afghan War of 1878-1880 they were more alike than they were different. British officers serving in both types of units but especially the Sappers & Miners often came from the Royal Engineers.
As Plynkes points out, both troop types could be and often were employed in a standard infantry role, where they usually performed very well, maintaining something of an "elite" reputation. For instance, at the Battle of Maiwand in July 1880, the half-company of No. 2 Company Bombay Sappers and Miners which were present were said to have fought very well under terribly difficult circumstances. Some gamers give Sapper & Miner companies bonuses for being elite Indian troops, as they would also give Gurkha and certain Sikh units.
If you do some Google Image searches you should find some photos and illustrations of both 19th Century Bengal/Bombay/ Madras Pioneers and Sappers & Miners. I myself have a unit of the Eureka figures you mention. They come in two versions, Sikhs and Punjabis -- who can also be used as Bombay or Madras Sappers & Miners, as well as Pioneers if you want to use them as such. The two different troop types carried pretty much the same tools and were certainly armed with the exact same rifles, be they Snyder-Enfields during the 1870s and 1880s, or Martini-Henrys in the 1890s.
As far as unit size for TSATF... well, that depends on how you think of your standard TSATF 20-figure basic units. If you see them as "Platoons" and are playing the game at more of a skirmish level, then you can certainly raise full 20-figure units of either Pioneers or Sappers & Miners, but if you see the 20 figure basic units as stand-ins for Battalions -- as some, including myself, have been known to do -- then you could raise full units of Pioneers but should stick with smaller size units of Sappers & Miners, since the largest unit they'd have would be a single company. For Maiwand I use a 10 figure unit of those Eureka Punjabi Pioneer figures to represent the half-company of Bombay Sappers & Miners, which I freely admit is WAY TOO MANY figures historically speaking, but since the British have the odds stacked so high against them that doesn't bother me. In truth, a smaller number, such as the 5 figures you suggest, or maybe 6, to represent a full company might work well. Right now I'm preparing the terrain and troops for a Battle of Charasiab game and plan to use 6 of those same figures to represent No. 7 Company Bengal Sappers & Miners. At the actual battle No. 7 Company was assigned as the infantry escort for two Gatling Guns which the British had with them.
Game-wise, Pioneers and Sappers & Miners are great for scenarios involving punitive expedition objectives for the Anglo-Indians such as destroying a Tribal tower or fort or some other valuable piece of terrain -- caving in a mine, burning crop-fields, destroying irrigation canals and flooding surrounding crops, etc. The only figures able to accomplish these tasks will be the Pioneers or Sappers & Miners, so the British commander should take care they don't suffer too many casualties before they reach the objective and are able to accomplish the mission.
Plynes is correct in saying that most of the time Pioneers and Sappers & Miners did not carry their tools into battle on their backs, but when the occassion called for it, they did. At charasiab for instance, the 23rd Bengal Native Infantry (Pioneers) started the day on a mission to prepare the road throught the Sang-i-Nawishta gorge North to Kabul so that wheeled artillery could use it. In that case they would certainly have taken their tools with them, though whether or not they kept them on their backs after the shooting started is another question. For myself, I like my Pioneer/Sapper unit with their picks or shovels on their backs, since it instantly identifies them as such, but I certainly admit that in purely historical terms, it's more likely that on the NWF or in Afghanistan those tools would have been with the pack mules and/or pack camels.