Are our white cloud minnows happy?

Question
We have a 27.73 litre (6.1 gallons) Clear Seal tank and a Interpet PF mini filter (says suitable for 5-40 litres) that's been running for about 2 weeks. 4 days ago we got 2 white cloud minnows and 2 gold white clouds and we intend to get 2 platys in a months time.

Q1- We are worried our filter is too strong, as the fish seem to often get pulled along sideways when they are staying reasonably still. They even go backwards when they swim to the back and surface of the tank where the current is strongest. Is this a game they are playing or are they struggling with the strength of the current?

Q2- Sometimes we've spotted the normal white clouds darting in at the gold white clouds, in what looks like an aggresive way. After reading around the internet, we've decided to get one more of each kind, as apparently they're happier in groups of 5 plus, but it would be good to know if this darting and chasing was normal playful behaviour or whether it was a sign of aggression (?).

Also, with 6 white clouds and 2 platys, will the tank still be big enough? Thanks for your help.

Answer
Q1- They are just playing

Q2- That is true. Yes they like bigger groups

Q2.5- Your tank isn't big enough for the fish it has already. I suggest upgrading to a 10 gallon. You could use all the equipment you have now, juse spend about $20 on a tank and $10 on gravel and what not. Then you could go along with your original plan.

10 gallons is the minimum tank size for any fish in my opinion. Anything else, and there is not enough room for any fish to swim, even if it is just a few small ones.

-Nick