all you could do is a trio of pgymy cories, they aren't suitable for new tanks though, get them now and they'll die fairly quickly. so leave it 2/3 months and then add them, plenty of time to do some further research into them.
be warned though, siamese fighting fish are not called fighting fish for nothing. temprements vary from fish to fish but some of them can be extremley agressive and will not tolerate any tankmates at all, so it's a case of proceed with caution and be prepared to return the cories if the betta is very agressive with them.
also you haven't mentioend what experience/background fishkeeping you have, as you've posted in the new to the hobby forum i'll assume none, apologies if i'm telling you stuff you already know!!
You'll need to cycle the tank before getting the betta, have a read of the links in my sig 'whats cycling' and 'fishless cycling' if you're not sure what this is. Also reading through the link 'step by step guide to setting up an aquarium' is a good newbie guide which will give you loads of pointers.
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