The lasgun is no longer an option for Astra Militarum sergeants indeed. They can get Ranged Weapons from the list as an upgrade, but the lasgun is not amongst those, and they don't have a lasgun option by themselves.
However, about tournaments, it would be wise to inform with the tournament organizer. For a friendly game I wouldn't mind if you mixed Vostroyans, Cadians, Valhallans, and even a few Catachan Jungle Fighters in a single unit. A guardsman is a guardsman is a guardsman is a guardsman is a .... Well, you get the drift. Considering the fact that all those regiments have a very distinct look, and some (like the Catachans) don't seem to have any armour at all, yet others (like the Vostroyans) seem to have very bulky clothes and armour with extra plates and collars seemingly suggesting Carapace armour... That might sow confusion.
I did read about your winter theme, and I don't see any problems there, but just to be safe, ask the TO beforehand.
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