As a newbie myself, I can offer some advice on what's its been like the last 3 months. I cycled with fish because I didnt know any better and my LFS sold me all the wrong for my 20g tank 1 common pleco, 3 red tail tinfoil barbs (I lost one while cycling so now I have 2) and 2 giant danios.
They also told me doing PWCs would stall cycling, thats about the time I lost my RTTF and found this forum. All in all my tank took 1 month to cycle and I had to do 50% PWC daily (you might not need to if your bioload is lower than mine was)
I would recommend that if you are going to cycle with fish you should definitely get a master test kit (ammonia, nitrite, nitrates and pH) and do a PWC every time ammonia goes above 0.5ppm (try to keep ammonia around 0.25ppm or less). Also I was only feeding my fish once every two days while cycling to keep the waste down.
Things I've learnt that I wish someone had told me from the beginning:
1)PWC will not stall cycling (I have read that ammo-lock and very high ammonia above 4ppm will)
2)Do not vacuum your gravel and clean your filter on the same day (this will take out too much of your good bacteria)
3)Unlike what the filter manufacturers want you to believe, you do not have to change your cartridges every month, just rinse in old tank water and put back in (I have a penguin biowheel 150 filter and keep two cartridges in there, I rinse the one in the front then take the one in the back which is cleaner and put it in the front every 2 weeks)
4)Stable levels in your water are as important as "ideal" levels (pH, temp, hardness and alkalinity). Try not to mess with these water parameters too much and if you do any changes they should be very gradual.
Here's a couple of links to threads I found useful
Common newbie pitfalls:
http://www.aquariumadvice.com/forums/f15/common-newbie-pitfalls-67468.html
Cycling with fish:
http://www.aquariumadvice.com/forum...-but-i-already-have-fish-what-now-116287.html
Regarding your fish, I think keeping the pleco is fine if you are planning to upgrade. Not sure about the cherry barbs with the betta but you can always keep a very close eye on them and take them back if your betta is very aggresive towards them.
Hope some of this helps, others feel free to correct me if I said anything wrong.