Can I mix brands/tons when installing AC unit to existing coils?

First off - the 'rule of thumb' is 600' per ton of Air Conditioning. In other words your old unit is a 2 ton unit. So - 2 tons x 600' = 1200'. As you can see if you install the 2.5 ton unit - you will be installing a AC that 'could' cool a 1500 sq ft house (2.5 x 600' =1500 sq ft.). Slightly more than what you need; and the 3.5 ton unit is 'way to big,' (3.5 x 600' = 2100 sq ft.).

Note: fyi - many in the AC business will sometimes refer to tonnage in btu's, i.e. 1 ton = 12000 btu - hence a '2 ton unit' can also be referred to as a 24000 btu unit and vice versa.

So... from the above - you can easily see that "2 tons" of Air conditioning is what is required to cool the 'average' home of 1100 sq ft. "roughly speaking."

Note: it is always best to have a professional 'size' your cooling/heating needs.

One of your questions was could you 'mix tonnage?'

The answer is 'usually you don't mix the tonnage of your outside/inside units.' However, professionals sometimes do (mix the tonnage) in certain situations, and installing a 2.5 ton outside unit with an existing 2 ton inside unit is often done, however, there are some 'tech issues' here and - I would "again" recommend that you call a Service Tech to help you with the sizing/mixing of your cooling/heating needs.

hope this has helped