If you already have 6-12-12 or a similar ratio, you can use it on roses for a while, but I wouldn't buy it for roses. Roses use nutrients in the ratio of 3-1-2. If you apply a 1-2-2 fertilizer regularly over the years, and if you have good soil that retains nutrients, then phosphate and potassium will build up in the soil. Eventually this buildup could cause problems with the uptake of other nutrients.

Nitrogen (first number in the ratio) is the nutrient most needed and also the nutrient that burns plants when used in excess. Therefore, labeled doses of fertilizer are keyed to the amount of N in that particular fertilizer. If you apply two different fertilizers according to label at about the same time, you will be applying a double dose of nitrogen. If most of this N is in soluble or readily available form, it will burn your plants. If most is in slow or timed-release form, it won't burn unless you reapply too soon. But generally, if you are using two fertilizers, the safe practice is to apply a HALF-dose of each according to the recommended time intervals.

You could do that with 5-1-1 fish and 6-12-12 granular, which would produce a better balance than 6-12-12 alone.

That schedule sounds like way too much fertilizer, but it depends on what kinds specifically you're talking about, and what kind of soil you have, what climate you're in and what time of year it is.

Usually when people apply liquid fertilizer every 2 weeks in addition to dry, they use a half dose or less on the liquid.

You don't tell us where you are (a pet peeve of mine! gardening is all about LOCATION) so we canNOT tell you what or how or when to fertilize without that location information.

I live in a short-summer climate with gravelly soil. Here is how I fertilize my roses. (the gravelly soil matters, because most fertilizers will wash right through it and be wasted.)

They get a slow-release organic fertilizer in November. Then in spring, whenever they have 4" of new growth (hasn't happened yet), they get another dose of slow-release organic fertilizer. Usually in May some time. Then a second fertilizing some time in June if I get around to it. One last fertilizing in late July or early August. That's it. Our growing season ends in September.

In California I fertilized every six weeks from April through October as the growing season doesn't end til December, and picks up again in February. I scheduled feedings after every bloom cycle, which is every six weeks, not once a month. That was heavy clay soil, so a loamier soil might want fertilizer once a month rather than every 6 weeks.

Roses need nitrogen primarily. Nitrogen is susceptible to washing away easily as it is very water soluble, more so than the other nutrients. Clay soils will hold on to nitrogen relatively well, but sandy or gravelly soils let it wash right on through. These soils absolutely require a slow-release form of nitrogen. You can supplement in peak growing season with liquids, but use a half dose as the full dose will wash through before the roses can use it.

I have about 130 roses and could never fertilize each of them weekly, or I'll go crazy: I'm alone in taking care of the garden.

I use Nitrophoska Gold, a slow-release fertilizer 15-9-15 which has to be given only twice a year, without being diluited: it looks like pink sand, and one just needs to throw a fist of it at the base of each rose. It melts slowly, with the rain and the humidity, keeping the roses fertilized for months.
I am very happy with it.