I DMed a game like this.

You could Gestalt with your previous class by spending two feats on them.

For example:

Rogue 6 has gained enough XP for 2 feats;
He takes a gestalt level in Wizard.
The d4 and the 2+Int skill points are not added because the Rogue gets better
He does gain the +2 to Will and spellcasting, as well as class features (familiar and scribe scroll, or acfs)

After 2 more feats the Rogue 6//Wizard 1 could take yet another level from a class. I did use fractional BAB for it.

They could also take PRCs as long as they qualified for them, though I did stop PRCs that gave them abilities too far from normal lv6


It worked out really well. Like, surprisingly well.