Actualy Annastasia I don't think I realy NEED to add to your reply. You answered that pretty fully. I will add a few extra things though...
Even just one kisser may be a problem because of their size but, otherwise, it COULD work out. That doesn't mean it will (especialy not with a fish that cannot be sexed accurately) but it might. Gouramies (like Annastasia mentioned) tend to vary in temperament. You can never be completely sue of how the individual fish will react to certain circumstances.
If you do go with the kisser, add the pearl gourami to the tank first, then get yourself a young kisser that is no bigger than the pearl but prefferably not any smaller either. Last, add the two three-spots at the same time about 2-3 days after. This way you'll give all the fish a chance to settle in according to their temperaments.
With the kisser, if there's only one, I would not be so concerned about aggression from it but from the aggression it may cause in the three-spots. Basicaly, because of its size, it will be taking up a considerable area for territory. Now while its territory probably wont be in much dispute because it will be the largest, it will be the 2 three-spots and the pearl that will suffer territory-wise. The result will probably be (if the three-spots are both male), that they compete for the remaining territory and force the pearl to retreat to some corner. If you plant the tank heavily, you can avoid this though. I suggest you do though you should be careful of what plants you include as kissers tend to nibble on a lot of them. Also, as you cannot sex the kisser, you will just have to rely on luck. If it's a male, it will be more territorial and may cause problems like what I mentioned above. If it is female, it will hopefuly be more mellow and will not bother anyone or take up as much space.
Are you set on getting kissing gourami(es)? There are many others you could try instead. A couple of banded (colisa fasciata) or thick-lipped (colisa labiosa) or moonlight (trichogaster microlepis) or any combination of 2 of those would work with your current gouramies (provided you give them ample space by not over-crowding with other fish, add them first to the tank so they can establish a territory before the other fish are included and prefferably have quite a few tall and floating plants in the tank as well). As I mentioned, adding them first is a good diea. If your pearl gourami is not fully grown or female, add it at the same time as the new gouramies as well.
Like Annastasia said, most fish that aren't nippy, don't get too big or are similar to gouramies themselves would make good tankmates. Consider filling the bottom layers first, catfish such as cories, small to medium-sized plecs, otos and so on or non-nippy loaches (but not clowns - they get too big) make good options. SAEs or a flying fox are options. Even a single RTBS or rainbow shark (but not with any simialr looking fish and that includes SAEs/flying foxes and other 'sharks'). You said you like variety, there are cichlids you could go with as a pair/trio (depending on species requirements) - keyholes are one of my favourites, rams (I preffer bolivians), some apistos and other neotropical dwarf cichlids, kribs and so on. Get only one pair/trio of cichlids for this tank though. Then you want to look at fish to fill out the middle layers such as barbs (non-nippy), colorful livebearers (watch the sex ratios), rainbowfish (stick to smaller growing ones like thread-fins or neon dwarfs as you'll want a good sized group so they aren't too shy), rasboras (harlequins are the very best schooling fish IMO - they swim so tightly packed), many of the tetras (black phantoms are my favourites) and pretty much anything else that won't nip and doesn't dwell in the top layers. Avoid danios, hatchetfish or similarly top-dwelling species. They will just take from the gouramies' space and cause problems (especialy if you do get lots of gouramies). Obviously, don't over-crowd or over-stock. One thing i can warn you against - do NOT get a CAE (chinese/golden algae eater/sucking loach). Besides the fact that they get big and don't clean up algae very well once adult, they get aggressive as they mature and are very likely to 'suck' on flat-bodied fish like gouramies and even take out their eyes and cause simialrly serious damage. Though they are a risk to all community fish, I felt it was worth a mention as gouramies are one of the groups of fish most vulnerable to attacks from them.
edit: For my terrible spelling.
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