Very new to all of this

Hi all,

Last weekend, we bought a large bay shaped tank which holds 150 litres of
water.

It has an under gravel filter with two "bubble chambers" (don't know the
correct terminology) -- one on either side at the rear of the tank. In the
tank we also have an air line thingy which just makes a pretty bubble wall
effect, about an inch of gravel, two large ornaments, two large plants --
still waiting for another large ornament which is on order.

We filled the tank with softened water (we live in a very hard water area)
and put water conditioner in and have kept guppies alive in the tank all
week as per the instructions from our local pet shop.

I went back today to get some real fish and ended up with 5 goldfish (the
small variety) and 3 molly's. The guppies are still in the tank as well but
the tank looks bare as though it needs more fish but we have the recommended
amount.

Do you reckon I could get away with adding more fish and what other fish
live harmoniously with goldfish?

I bought two male siamese fighting fish for the kids but both of them are in
separate tanks in their bedrooms.

Would I be able to put female fighter fish in the main tank with the
goldfish?

thanks in advance. I look forward to learning lots more on this subject!
A