Read the label on your potting soil to determine whether it comes with fertilizer already added. If not, it is helpful to fertilize as soon as the plants leaf out. I would use a soluble fertilizer like Miracle Gro at standard dilution or at half strength. Either way, it will not hurt the plants unless you use more than the label says to..

Superthrive is not a fertilizer, and as far as I know there is no evidence that it has any effect on plants.

There probably is fertilizer in the soil already, most of them have it now days, but you can use organics anytime, which is what fish emulsion is, and I always use a little Superthrive on my new roses. Like Michael says, there's no proof it works but it can't hurt either!

Forgot to mention....I use a premixed soil of aged mushroom compost, bark fines and sand that comes in bulk from a local company - get it by the pickup truck load. My plants thrive in it. For newly planted roses, I add vermiculite or perlite. I have on hand Mills Magic Rose Mix, fish emulsion, Superthrive.
Judith

Go ahead and use fish at 1/2 strength every 2 weeks for three times. Then use MMRM after they are going decent. That mix you have your roses in sounds great. Wish I had something like that up here.