Looks like you guys wanted to know how Synthetic Beasts are constructed what the rules are. Let's state the two key things that will help with make these guys.
Egg Groups and Typing are your tools of creating a stable fusion.
Each fusion can contain a large amount of pokemon if the egg groups and typing match correctly. Think of it like slots, if you have a mono type pokemon you have to match a pokemon that has that typing as well. For example you have a mono water type that means it can only be paired with a pokemon that is also a water type or have water as a secondary type. If you do that then the two water tapings connect and give you an extra slot plus a new type possibility. If you took a pokemon that was a water type and paired it with a pokemon that doesn't have the water type but is either from the same egg group or connected to that egg group that the fusion is liable but you can't add more pokemon to that fusion.
Egg groups also help give you slots, it all kind of like a matching game, if the pokemon is from the exact same egg group as the paired pokemon then you get another slot. This means if you match a type and an egg group you get two more slots which means more pokemon right? Well not exactly, the slots act like a safety net for if you add a pokemon that has the same egg group but not the same typing or vise-versa but it's the typing that take up those slots which means put a pokemon with duel typing and your two slots are gone and the fusion can't go any further.
Synthetic Beasts can also mega evolve but only if all the pokemon within the fusion or a large amount of the pokemon in the fusion can mega evolve as well.
With all of these factors at play this allows for a smooth and complete integration of all parts of each pokemon, it's like creating a hybrid or breed of new pokemon. This means that there isn't just one pokemon show casted in the design as a whole, you can pick put specific parts of where each pokemon exists in the fusion yet it still look like it's own kind of mon.
To figure out what the fusions typing is you have to look at what are the most recurring types in the fusion. The types that pop up the most are used for the typing of the fusion, but what do you do if you have three recurring types? Well look at the fusion and what you think it can do then you got from there. You have water, bug and flying as recurring types, if your pokemon doesn't look like it can fly then it's a water bug type.
From well with all that explained you guys can make your own synthetic beasts, enjoy making them ^u^
