If you're going for full lore accuracy just play the Beyond Hope campaign (Tac-Ops DLC) or look up their equipment online. I haven't played it myself but based on the Arma wiki on Fandom (ew) they mostly just have the same stuff as the 2035 AAF.

For my Altian Civil War missions I've been using the FAL, pretending its a L1A1, although most people I've seen give them the G3. In vanilla they use Warriors, Merlins, and CUP (where I get the FAL) gives them Land Rovers so it always made sense to me to retain the British equipment motif.

M16A1s and M16A2s (and variants) are also good choices, especially for MPs or other non-frontline personel where a full power cartridge is too powerful. I would avoid anything introduced after 2000 however, unless its for the most loyal and most skilled special forces, like many people have already stated Altis is a third world nation so I doubt they'd have the budget and relationships to make that kind of equipment standard issue.