Well thats a hard one to answer really, my fast growers are still in my tank after about 7 weeks, but i had some slow growers in there as well from the beginning, particularily my foreground planting which is taking forever to fill in, but i would guess that about 60% were fast growers 20% medium growers and the rest slow growers, and i planted heavily from the start, the pics that i put up in the picture section, if you look at the first picture, it shows my tank on day one, but after taking that shot i added even more plants.
In the beginning i just made out a basic design, back, midground and foreground, and gave each of the plants enough space to what i guessed they would grow to within 3-4 weeks, and i covered pretty much all of the substrate this way.
I think its no harm to not get really good plants for a good while anyway, because you will have problems, thats a certainty, you will make a good few mistakes in the beginning as you juggle with your co2 ferts etc, and it does take a while to really get into the swing of things, im useing the EI method so that was a big learning curve in itself, but i would say at this stage im getting quite comfortable with the logistics of manageing the tank, but in the beginning its tricky until you have a few weeks under your belt.
You will most certainly get a diatom bloom within the first 2-3 weeks and this will leave your beautiful tank a mess and your plants as well, ottos are very good for this so i would get some into the tank early on, after i got the diatom bloom which lasted about 2 weeks (and believe me the tank was a mess) i got some green hair algae which lasted for about another week, but this soon disappeared and hasnt been back since, but it will definatly take you a good few weeks to get into the swing of things, for the first few weeks its a bit hit and miss until you get a bit more experience.
As regards the fast growers they recommend that you leave them in for about 8 weeks in a new tank, and in my experience so far this would seem like exactly the amount of time that you would probably need to get the tank established, i wouldnt go crazy on pruneing in the first 4-5 weeks, just do the minimal to make it look ok, and id say at about 7-8 weeks you could start making bigger changes, but id still be slow to make dramatic changes at this stage because you will throw the balance of the tank off again, and it takes a bit of time to understand the new balance as it were.
Well that was a long winded answer, but thats pretty much how it has been for me up to now.
EDIT: One thing i forgot to say is do loads of tests in the beginning, im sure i dont need to tell you this, i would say i was testing nearly everyday for the first 4-5 weeks until i figured i had got a handle on things (but im useing diy co2 so at a disadvantage to yourself, so i had to keep testing for this) but i dont test as much now, maybe only a few times a week usually just to check the co2 and the odd nitrate and phosphate test.