It sounds like your seedlings are "leggy" though I'm not sure without a picture. If this is the case, it's an issue of inadequate light, not fertilization.

Are you talking about tomatoes specifically?

Don't cut them off. Buy some nursery pots and pot them up while you're waiting to put them in the ground. They will be bigger and stronger than if you plant them directly from your starting kit.

Just in case you haven't read this elsewhere - when you pot them up, bury them deep (almost up to the lowest set of leaves, you can even snip off the lower leaves if you have a lot of leaves and then bury to the lowest set you leave (no pun intended)). How many leaves do they have?

15"? WOW! When did you start them? I would think it's partially stretching for light, and partially that starter fertilizer. Do you have any grow lights? Putting the repotted plants under artificial light will help. Putting a fan on will strengthen the stems.

What zone are you in? Can you start putting them outside for part of the day (almost like hardening them off)?

Just to answer your specific question about trimming the plants back, I wouldn't do that. You want to keep the main stem growing because it will produce the biggest and earliest tomatoes. Some people top off their tomatoes near the end of the season, but never early in growth. It's better to shorten the plant by burying the stem much deeper.

No problem at all when they get tall like that. Deep planting works.....and the OTHER, just as good approach is laying them down sideways and bending up the last 4-5 inches above ground.