I think in almost all cases it works out just fine for a male/female split tank. You should have a backup plan just in case, but you probably won't need it. If you get the Penn Plax divider they do go right to the top but you should get a glass canopy to put under your regular hood (or in place of your hood) otherwise there will be a small space right where the feeding door is where the female can jump through. With the glass canopy the divider fits flush with the divider and there is no space. The canopy is cheap (around $10 or less) and you can just put your light strip right on top of it and not use the plastic hood at all.
When I put the divider in Lava's tank he freaked out for about 20 minutes, not about Karma being there but about the divider. He's a very sensitive, high strung fish and doesn't like change but he did get used to it and enjoyed Karma's company. Lava is now in the 5 gallon Hex by himself and is doing fine. One of my 10 gallon tanks is divided into 3 sections and I have 3 of my girls, Aenara, Angel and Noelle in there. They get along great (as long as they can't bite each other) and interact with each other constantly. My other 10 gallon is divided into two sections with Phlox and Whisper. Phlox is such an easy going fish! He is just fine no matter what I do. He was attached to Karma and when I had to remove her to put her in Whisper's tank he sulked a bit. He got over it pretty fast as Whisper is so active, teasing and playful he couldn't resist her! They play together all the time and are so much fun to watch. Sometimes I catch them kissing through the divider or just racing back and forth together. Phlox was pretty lazy before he got a roomate and not getting enough exercise. Since getting a girlfriend he's constantly active, swimmy and showing off his fins! One thing I've noticed is the females are a LOT more active than the males! Having a female next to a male makes the male more active too.
They are very competitive and jealous at feeding time, all of them! I try to drop the pellets quickly into each of the sections so all can eat at once but most of the time they are so worried about what's going on on the next section they don't see their own pellet. I have to holler at them all the time and point out that they DO have a pellet in their side just like the others!
The girls tank is the funniest! All three have different behaviours at feeding time... Aenara is my jumper! As soon as I open the feeding door she starts leaping out of the water. She won't even wait for me to drop the pellet. She is so impatient she jumps up and nips my fingers trying to get it. I try to drop hers in quickly first so she stops jumping. Angel is next to her and she wildy swims back and forth along the glass and doesn't pay attention to the pellet and usually knocks it away with her head as she zooms back and forth. Then there's Noelle, who stands in the corner and furiously wags her tail so hard she churns up the water and causes such a wake she loses her pellets when they float away.
Feeding those three takes the longest as I have to get them settled down first so I can get them to pay attention to what's being dropped!
Divided tanks can be a lot of fun! Having the fish together (but protected from fighting each other) really seems to bring out their odd little personalities even more and keeps them active. You just need to be very careful that they cannot jump over and get to each other. You also definately need a hospital tank as when one gets sick, unless it's contagious, you cannot medicate them in their tank with the other healthy fish.
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