Hi,
I can't advise on the filtration aspect as I know very little about under-gravel filters; however, others will be able to help with that aspect. I am fairly certain that I've read that under-gravel filters aren't a wise choice if you were planning to keep bottom feeders (eg corydoras), though.
In terms of 'bacteria in a bottle', I did use Tetra Safestart, with no adverse consequences, but I think others have mixed views of whether or not these are beneficial and worth the money.
As for stocking ideas, tetras are ideal for this size of tank - but tetras should be kept in shoals of at least 6 of the same type of tetra (eg 6+ x-ray tetras as opposed to 3 x-ray tetras and 3 cardinal tetras) and seem to do better in larger groups. You could 'comfortably' have a couple of these "half a dozen" sizes of shoals - in fact, if there were one particular type of shoaling fish that you particularly liked, then 12 of them would actually fare better in terms of group behaviour and intensity of colouring than two different shoals of 6 as it's more akin to their natural environment. The 'community creator' on this website
http://www.thinkfish.co.uk/fish-profiles is great for putting in details of what you might like, and working out what might or might not be feasible for the size of the tank - you could request that it only provides details of fish under 5cm, for example.
What I'd strongly advise is to find out what your water parameters are, so that you can work out what type of fish would suit that water. For example, if you had very soft, acidic water, then you'd be best to avoid some types of fish that are more suited to hard, alkaline water. Once you've established that, then it's looking on the 'community creator', and then browsing round some aquatic shops to see if anything particularly catches your attention and 'matches' what you might have seen on the 'community creator' as potentially suitable - and 'cross check' online once back home as sometimes there are similar sounding names but actually very different fish. Also, sometimes the labelling on tanks in shops isn't accurate because fish have been moved about, so worth going in with an idea of what you're expecting the fish to look like, then, when you've found them, see if the fish are indeed to your liking or not.
Hope this helps, and keep us posted on what takes your fancy, and we can advise from there re potential compatibility of other tankmates, etc.