Agressive Gourami

I have a community tank with cardinal tetras, harlequin rasboras, 2 plecs, 2 yoyo loaches, 1 Platy, 1 Molly, and I Banded Gourami. I did have several mollies, 2 Platys, and 2 Gouramis. But over the last year some have died. I added the Gouramis last August. In the Autumn i noticed my last remaining Guppy had Lost it's anal fin and most of it's tail. We don't know what happened and about a month later it died - well, disappeared, I never found the body. Over the last month I have noticed that the gourami spends a lot of its time chasing the molly around the tank, sometimes at quite a speed.  I didn't think that Gouramis were supposed to be aggressive. Could it have been that who attacked the Guppy? Why is it pestering the Molly? and is there anything I can do about it.

You might also look at your yoyo loaches as possible suspects with regard to your guppies. They are known nippers and are not really recommended as community mates for guppies. Both they and the plecs are also really "big tank" fish, needing even more space than the gourami.

Gouramis belong to the Anabantidae, together with bettas and paradise fish among others. Many of these are essentially timid fish which like their own space, a quiet place where they feel safe. If you have a very open tank, with few or no plants (real or artificial) it is possible that the gourami feels constantly exposed and responds by aggressively chasing other fish. "Out of sight, out of mind" is far more true for fish than for people! It is generally recommended that you supply "broken lines of sight", i.e. plants or other decoration arranged such that fish can find places where they are out of sight of other fish, where they will be more relaxed and so less aggressive. Gouramis often stay near the surface and floating plants to shelter under might be useful.

Although you say you have a community tank, it seems to be a bit of an odd mixture, with fish which are not that likely to get on with each other and/or really need different conditions (e.g cardinal tetra and molly).

Now that I read your post more closely:

I agree with Richard about the loaches. They also need to be in a shoal of at least 5, with more being better. And a 5 foot tank. As with most fish, if they are not kept in conditions they like (eg not enough of them) they can be aggressive. It could be that the loaches have only recently reached the size where they have begun to feel cramped, and their behaviour then changed. Now that I've noticed you mentioned the loaches, they are the number one suspect for the disappearing guppies.
And if the loaches are upsetting the gourami, that could be attacking the other fish because it is stressed.

The reason that Richard said that cardinals and mollies do not go well together is that cardinals much prefer soft acid water while mollies like it hard and alkaline; some types of molly can even live quite happily in full marine water. Mollies can develop a condition called the shimmies if the water isn't hard enough.
Platies also like hard alkaline water, as do most of the common livebearers.

Richard is a plant expert; listen to him on the subject!