I can see from a beginner’s point of view, hearing about the need to feed tomatoes, that you could be persuaded to over do it.
Yours already have more than enough compost, in fact my last few plants from a Feb. sowing have their first flowers and are in 4" pots.
I think virus is unlikely, perhpas just some dodgy seed, there are always a few that don’t make it.
Probably no feeding until July at the earliest.