Using gold increases weapon prof and STR/AGI, not skills.
That said, you could (I think) boost the knights Stats/WP above that of your trainer (Trainer WP+30), raising the cap for the sergeants (which will now be trainer WP) such that the trainer can raise their stats/WP to what the trainer has.
Might be annoying though. If for example your trainer had 450 archery WP (also the cap for your sergeants) and the sergeants had 449 WP, then the percentage chance per day of them getting to 450 is almost Training/3 -0.5 %. So our elf friend who has 0 training will have no chance to raise them that far... If you somehow could get him a couple of levels, throw it into INT and then put the points into training you might be able to coax the last few points out...
Also, I guess you could really specialize and have one trainer (with a little training of course) put every single point into strength and another into agility. Ideally the ones whose (Stat-level) is the lowest, so you can push as far as possible to get the bonuses for them. Since Weapon Master 10 gets you to 420, you could possibly get the AGI person close to 450 or even higher for their WPs. (I think one of the knights has good melee proficiencies though, and comes with trainer). However, I don't know if the weapon master skill helps the soliders.
Still, it would be amusing to churn out sergeants with above noldor stats (Rangers have 400 in all profs). I think there are uber-heavy armors with more defence, you can possibly get their WP up and then they can have 9 powerdraw. And double bags of noldor arrows...
You'd have to grind like mad to actually get anywhere near the Noldor Twilight Knight's 600 weapon proficiencies though. Even weapon master 10 gets you to 420, getting 180 more would take forever.
