The way I see it, The Mountain will have a small force of dedicated retainers, but they would pretty much be mercenaries, brigands and other 'scum' who have come to work for him full time. They certainly wouldn't wear uniforms. In fact i've seen a lot of (very good) fan artwork of Gregor and his men, and they are usually wearing yellow surcoats with the Clegane arms on, which never seemed right to me. I doubt he would go to the effort of getting uniforms made up to give to his new recruits. And I doubt he'd be seen wearing his own heraldry much except perhaps at a tournament.
Notably in the book, his men refer to him simply as "Ser" (i.e. Sir). They do seem somewhat loyal to him, as far as he is alive and well. After his little incident with The Viper, most of his men are hanging around bored at Harrenhall until Jaime Lannister turns up, and they join his army. They are probably thinking something along the lines of "Oh well, I don't think Ser's coming back, what shall we do? Go outlaw? Meh, let's join Lannister's army, we'll have food and shelter, but we might have to behave ourselves..."
Actually, a favorite bit of dialogue in the book, when Jaime arrives at Harrenhall and speaks to the one called 'Shitmouth'....
SHITMOUTH: Fuck me, Jaime Lannister... It's the bleeding Kingslayer, boys. Fuck me with a spear!
JAIME: Who might you be?
SHITMOUTH: Ser used to call me Shitmouth, if it please m'lord.
JAIME: Charming. Do you command here?
SHITMOUTH: Me? Shit, no. M'lord. Bugger me with a bloody spear.
JAIME: You heard the man. Find a nice long spear, and shove it up his arse.
And a few pages later, when Jaime gives them orders...
JAIME: Until such time as Lord Petyr arrives to claim his seat, Ser Bonifer Hasty shall hold Harrenhall in the name of the crown. Those of you who wish may join him, if he'll have you. The rest will ride with me to Riverrun.
MOUNTAIN'S MAN: We're owed. Ser promised us. Rich rewards, he said.
SHITMOUTH: His very words. 'Rich rewards, for them as ride with me'.
SER BONIFER: Any man who remains with me shall have a hide of land to work, a second hide when he takes a wife, a third at the birth of his first child.
SHITMOUTH: Land, ser? Piss on that. If we wanted to grub in the bloody dirt, we could have bloody well stayed home, begging your pardon, ser. 'Rich rewards', Ser said. Meaning gold.
Does make you wonder what life at Clegane's Keep would be like... Not very pleasant I imagine. You wouldn't want to work there as a servant. During peace time, it would just be Gregor and his men sitting around torturing people I imagine....
If you were the sort of scum in Westeros who enjoyed murdering, raping and torturing, and wanted a bit of full time employment and a roof to sleep under, you'd probably just go seek out The Mountain.
As to modelling them, surely this would be a great opportunity to make some brigands / mercenaries with no real allegiance? Great for characterful conversion opportunities... No uniforms, a mix of weaponry, and a real scummy appearance. If you get round to making The Mountain himself, they can become Clegane retainers, otherwise they can be used for all sorts of factions.
As to The Hound, I see him as a total loner with no real loyalty to his House. I doubt Gregor has kept his bedroom at the castle for him! I can't imagine The Hound ever wearing Clegane heraldry or leading Clegane troops. He is just a fighter, and during the events of the books / show is effectively employed by the crown as Joffrey's bodyguard and then a member of the Kingsguard, just through his connection to House Lannister. He 'leads' troops at the Battle of the Blackwater, but just in terms of fighting, not actually commanding or anything. As soon as it looks like things are going against them, he deserts and becomes a lone wolf looking out only for himself. If he hadn't had the bright idea of taking a Stark girl for hostage money, he would probably have ended up either a lone outlaw, perhaps joining a group like the Brave Companions (but I doubt he's really sadistic or cruel enough to get on with them), or more likely cross the Narrow Sea to find work as a mercenary.
The point is, when it comes to wargaming, I don't see him as part of the Lannister faction at all, just a very interesting loner character.