Well sorry to rain on the parade but there are a mass of differences between the Sikhs in the 1840s and those in the 1850s in terms of appearance of uniform. I would suggest you take a look at Michael Perry's artwork in his Osprey Sikh Army book to get an idea of things.
All that said, if you want to use them for both the Sikh Wars and IM then who is stopping you...I am sure Mike at Iron Duke will happily take your money...but whatever you do, don't waste your time with the Studio Minis Sikh War range, as for the most part it is not good at all....uniforms are fairly accurate but the figures aren't that great, riders don't fit properly on any horses, etc.
The two best sources for actual Sikh War figs remains Foundry and Old Glory. The OG Sikh Wars range (which was never completed) is actually pretty good, and the figures are very compatible with the Foundry ones. I should know, I have loads of Foundry and some OG in my collection. All good stuff.
Hope this info helps.