they should be ok in theory, ottos tend to do better in mature tanks with plenty of algae and a constant supply as they tend to be difficult to get on proper foods.
a way round this however would be to have a series of rocks, leave them in a tub of water in the sun and the aglae should grow, you can then keep alternating these rocks giving you that constant supply
Otos mix well with bettas but they are a more difficult species to keep. Firstly they are POO MACHINES!!! Youd really have to be on top of your water quality and have loads of live plants. Otos need constant supply of soft green algae and many starve if not in the correct setup. You can suppliment their diet by using rocks placed in a shallow container with water and leave it on a sunny windowsill. Soon green algae will grow on the rocks and you can add one to the tank to feed the otos. When its all clean just replace it with another algae covered rock (have to have a steady supply)
also, otos tend to do best in groups of 4+...not enough room in your tank im afraid. As for siamese algae eaters id say no. Also poo machines and not really enough space.
Endlers or shrimp are likely your best bet. You could have around 5-10 cherries if you have lots of hidey holes/plants for the shrimp to feel secure. They add very little to the overall bioload of the tank
i have just read through this whole conversation and, in my opinion, some shrimps and a betta are definitely your best bet. hexagonal tanks can be a nightmare to stock!! generally fish prefer rectangles as the increase the surface oxygen and provide more swimming space. the minute you go for any other shapes, you automatically rule out fish such as corys, danios, harlequins etc. so yes, i think you have made the right choice for your tank. check out the beginners threads on cycling etc. they will help you loads. if you really get into the hobby and decide at any point to get another tank, do try to buy a standard rectangular tank...it will seriously increase your stocking options!! for now though, i think the tank you have will be great with the bettas and shrimp.
ps - i have been to malta 3 times on holiday. it is a lovely place!