Here's my 2 cents, based on experience towing an A/S with a Chevy 5.3. (2000 Chevy 1500
Silverado with a 19' Bambi)
The 5.3 is a great engine. The only drawback (for towing) is that in order for it to perform you have to spin it! Since your Burb has 4.10 gears this should be no problem as long as you tow with the trans in the "tow/haul" mode. You will need to lock it out of O/D whenever you're actually in the mountains otherwise you'll find yourself in O/D at the bottom of a steep hill and will loose momentum so fast the trans will end up in 2nd gear in about 1.5 seconds (speaking from experience). I towed my Bambi north out of Durango on hwy 550 and never had any trouble accellating uphill even at 10,000'. I averaged 14 MPG towing my Bambi from CA to CO and back and spent very little time on the interstate.
Your assumption of subtracting your Suburban's GVWR from it's GCWR is a good "quick & dirty" method of determining max trailer weight. However it ignores the fact that most of the trailer's tongue weight is carried by the tow vehicle and needs to be subtracted from your payload. Also, you were adding tongue weight to the trailer's dry weight which is already figured into the dry weight. If you add 1,000 lbs to the trailer's "wet weight" you'll come pretty close to your normal road weight (based on how much weight we carried in our camper with our son and 2 small dogs).
Since you have a 1/2 ton Surburban I'll make a recommendation regarding tires. If you have 265/70-16s consider replacing them with a set of 245/75-16s. GM mounts the 265/70s on a rim that too narrow for best stability.
As for whether a 25' is large enough it all comes down to what you can live with. The only advantage to a 27' Safari (over a 25') is the split bathroom, larger wardrobe, and a bit more space in the living room area. There is only about 500 lbs difference between the 25 and 27 foot Safari.
When our son was 6 months old we took him on a 3 week trip in our 10' slide-in camper along with 2 small dogs. Very tight but doable. The trick is setting up a sleeping area just for the baby that you know they can't get out of. How are you going to bathe the baby? The cramped quarters and bathroom configuration is what convinced us to sell our truck & camper in favor of a trailer. We are convinced that a 25' Safari will work well for us as soon as our son is out of the "terrible twos".