A small shoal of Cories would do fine in a ten gallon. That's what I have with my Betta boy n_n Though a small species, like Pygmy Cories is recommended. Mine are a bit larger so I'm a little overstocked, but I keep up with water changes well enough that they're doing pretty well. I recommend getting small ones if you can, though. They should be in groups of about 5 at least, though. People say 3 is good, but I honestly don't think that's enough. Ghost shrimp are good little cleaners too, but make sure to get large ones or they will quickly get eaten up by an adult betta.
For a heavy planted tank ghost shrimp are great to eat the dead plant matter, they keep my ghost shrimp tank ( yes I have a 10 gallon breeder ghost shrimp tank LOL) completely devoid of any dead leaves and plant matter, never need to vacuum dead plants out, they eat it all but do not touch algae of any kind that I have noticed. They love any type of food. I'm completely in love with shrimp!
I love cories and recommend pygmy cories as they are fun little creatures. I have 2 african dwarf frogs who are the funniest things ever and Bonus they eat my duckweed off the top so it never gets out of control. Those are the 2 species that are just FUN, they are so happy and cute I'm totally in love with watching them swim around.
Nerite snails/amano shrimp are good options. Mystery snails and ramshorn snails.
I have mini yellow rabbit and orange poso snails in one tank and they are by far the most beautiful snails I have ever seen. They have the cutest little faces and will eat the heck out of a carrot when I put it in the tank.
Platies do a decent job of eating some algae and keeping the plants over all clean, a boy and girl Mickey Mouse platy are the cutest things ever together. Mollies get to big but also clean, I wish we wouldn't have bought the mollies as they aren't really nice and have gotten almost 4 inches long in our big tank, their faces just weird me out when they look at me.
If you are after an algae eater - otos and bristlenose plecos are supposed to be some of the best but they are also POOP MACHINES as are snails. Nerites are pretty cool snails but they are small - about dime sized. Apple snails are also pretty neat but they can get big. I got mine this size----> O and now they are about 2 inches. I think otos are supposed to be kept in a group?
if it were me, I would go with the Bristlenose pleco but that may require bigger/more frequent water changes and gravel vaccuming.
It has a well-deserved reputation for eating algae, and the aquarist must ensure there is a steady supply of either algae or vegetable foods, or the fish may turn to devouring plants. Bristlenose plecos require some driftwood fiber in their diet (though not to the extent of Panaque sp.). To ensure healthy plecos, it is recommended that at least some driftwood be included in the aquascape. The tank should be well-planted with pieces of bogwood and caves made from rocks or clay flowerpots; the substrate may be of smooth gravel or sand. Floating plants are beneficial to lessen the light. Like many plecos, Ancistrus can be largely nocturnal. They will be more active during daylight hours if kept under dimmer lighting or with a good cover of floating plants.
When netting these fish, care must be exercised to not entangle the defensive spines positioned on the sides of the head and the pectoral fin spines in the net; a long jar or container is preferable to using a net.
bristlenose and ancestrus(spell?) plecos get to about 4 inches full grown so it could be housed in a 10 gallon if you have driftwood, they really need driftwood to be happy as they actually munch on it. I moved mine to my 29 gallon after it got to about 2 inches because I wanted to add something different to my 10 gallon. After they get full grown it's a toss up as to whether they continue to devour algae, most people swear they don't do as good of a job after they get older and full grown.
Shrimp would all depend on your betta. Some bettas can live with shrimp and not eat them or the shrimplets and some bettas will kill all shrimp on sight. Every betta is different in that.
I've decided on Amano shrimp for long term cleaners for my 10 gallons, as they eat all kinds of algae and are similair to ghost shrimp, which none of my bettas have an issue with at all.