I've used my Golem Workshop fodder leveling pair (per this guide: https://www1.flightrising.com/forums/gde/1666709 which might be slightly outdated) for double-duty in FP since it was released. It's probably not maximally efficient in terms of stats or element balance - my mage is Wind and my fighter Nature, which can cause some nasty interactions with some of the Portal mages - and I might make an extra pair just for that venue someday, but they served well enough while I was collecting familiars early on and getting fest loot during the Arcane holiday.
Re: your second question, I've got nothing. I trained up my first few Lv 25s entirely slapdash and long enough ago that I don't remember much, balanced stats on the first one until after I'd gotten her fully leveled and had to tincture her. Since then when I want a 25 with a new build, I toss it on a team with one of my current trainers (usually rotating normal fodder through the second slot) until it's at least Lv 20 - sometimes the intended venue is forgiving enough that you can less-efficiently train there with a dragon who doesn't have their complete stats.
A big problem is many guides give you just enough STR when fully statted to rallied-eliminate the toughest non-boss in the venue, and since rally is dependent on the level of the dragon casting it, you can fall short on some enemies even if you have enough stat points to reach the recommended STR (and you don't want to go over that or you'll have to tincture later on) by skimping on QCK and VIT.
So it's a question of whether the satisfaction of being able to level up a trainer and trainees at the same time is worth the extra effort (restarting more often, etc). Having the dragon start to function as a fighter from a lower level seems much harder and, again, really only something I'd do for the fun/interest of it. More efficient to pack levels on them as you would a fodder until they can pull their weight at least somewhat in the higher venues.
Re: Forbidden Portal Teams?
Re: your second question, I've got nothing. I trained up my first few Lv 25s entirely slapdash and long enough ago that I don't remember much, balanced stats on the first one until after I'd gotten her fully leveled and had to tincture her. Since then when I want a 25 with a new build, I toss it on a team with one of my current trainers (usually rotating normal fodder through the second slot) until it's at least Lv 20 - sometimes the intended venue is forgiving enough that you can less-efficiently train there with a dragon who doesn't have their complete stats.
A big problem is many guides give you just enough STR when fully statted to rallied-eliminate the toughest non-boss in the venue, and since rally is dependent on the level of the dragon casting it, you can fall short on some enemies even if you have enough stat points to reach the recommended STR (and you don't want to go over that or you'll have to tincture later on) by skimping on QCK and VIT.
So it's a question of whether the satisfaction of being able to level up a trainer and trainees at the same time is worth the extra effort (restarting more often, etc). Having the dragon start to function as a fighter from a lower level seems much harder and, again, really only something I'd do for the fun/interest of it. More efficient to pack levels on them as you would a fodder until they can pull their weight at least somewhat in the higher venues.