Plasmas will cut pretty much all the alloys you're likely to come across. Presumably we're talking aluminium alloys?
Plasma cutting produces a heat affected zone which can locally alter the properties of many alloys. Aluminium cut with plasma + air should have this HAZ removed (sanding/grinding etc), at the very least it'll make welding more difficult and in the case of harder, heat treatable alloys the HAZ can contain micro cracks.
As Weldequip said, won't cut it's rated thickness with Al alloys due to the nature of aluminium
many thanks guys , i think ill drill the holes then and use a hole cutter , then sand alloy weld the joints to be safe ,,, glad i asked , many thanks for the advise