Aloha!
1cm=1 yard is good for 28mm. Personally, I use 1/2"=1 yard on the rare occasions I play Inq28.
Using preset characters is a great idea for introducing players to the principles of INQ and, if you're going to do a 28mm campaign with novices, you might want to do the same thing again (the last 28mm campaign I played included a SM kill team, a daemonic incursion and a brood of Genestealers until I stepped in and banged some heads together). The sheer freedom of INQ can lead new players to tool up the most monstrous of characters.
The other option, should you see this sort of outcome looming, is to design a campaign that favours the subtle and balanced. I find having power armoured sentinels falling through wooden floors, or being forced to remove their armour to climb through narrow tunnels, is good for imposing a degree of sense and humility on players.
Also, don't underestimate the preparedness of players to go 54mm. It's cheaper than many people expect, gives a better game experience and some great hobby challenges.
R.