I really like [name]Leo[/name] [name]Maximus[/name] for a boy but is that too gladiator sounding?
Other options I like are [name]Leo[/name] [name]Xavier[/name] and [name]Leo[/name] [name]Knox[/name].
I think any of the above are great.
I don’t find [name]Leo[/name] to be ott. You could tone it down even more with [name]Maxwell[/name], or maybe [name]Maxim[/name] would be a good middle ground. I like the [name]Max[/name]- names more than the other 2 you mentioned.
[name]Leo[/name] is most definitely not over the top. I’d say anything in the top 1000 would not be. As for not in the top 1000, anything that most people cannot pronounce would probably be ott, but there are exceptions. Then names like Honeydew, [name]Sunshine[/name] or Sweetpea. And names like Theodorius and [name]Hannibal[/name].
The best way to know is to ask other people, but ask A LOT of people. There will always be some people that think an unusual name is weird, but the best way is to poll a number of people. I posted a thread recently asking if the name [name]Fable[/name] was usable. I got a few strong negative reactions in the comments, but in the poll, the majority thought it was usable. The negative people always tend to be the most vocal. 
As for Leo Maximus… it’s totally normal, not over the top at all.
I would not consider [name]Leo[/name] [name]Maximus[/name] over the top.
I think “over the top” really depends on personal opinion. It’s very hard to find a name that I would personally consider over the top, I am totally OK with sounding like a gladiator or a princess. But I would say if it sounds like a My [name]Little[/name] Pony or it’s getting over 5+ syllables, it might be over the top.
[name]Leo[/name] isn’t over the top, it’s quite common.
And honestly, I think it’s pretty dang difficult to throw at me anything I’d consider [name]OTT[/name] for the middle name spot.
I think of it basically a free for all as long as you don’t use Stupid or Hitler or Vacuum.
My favorite of the ones you listed is [name]Leo[/name] [name]Knox[/name], but that’s mainly because I’m not a fan of [name]Max[/name] (or it’s variations) and it would irritate me that some people pronounce it eks-avier
[name]Leo[/name] [name]Maximus[/name] is not outlandish at all. [name]Leopoldo[/name] [name]Maximilian[/name] might be ott. Or [name]Lion[/name] [name]Maximo[/name]. You get the idea. [name]Leo[/name] and [name]Maximus[/name] are both quite in sync with today’s naming trends. [name]Leo[/name] is def in the top 1,000, I would imagine in the top 200, and [name]Maximus[/name] not far behind.
Here’s a good litmus test: If a celebrity named their baby [name], would the media declare it “another wackadoo celebrity choice”? I feel like if, say, Adele’s kid had been named Leo Maximus, we all would have yawned (not because it isn’t a solid name, just because it’s pretty tame). Obviously this has a fairly limited application, since celebrities are trendsetters and names like Phineas and Seraphina, which were once considered pretty out there, are now relatively mainstream.
You can also just surf some baby naming blogs and check the SSA stats. It will become clear fairly quickly which names are thought of as too out there, even for liberal-minded Berries. Leo Maximus? You’re fine. Lionel Maximilian? A little flamboyant, but okay. Leonteus Maxi Pad? No.
The nice thing about the middle name spot is that its there but hidden each and every day. My oldest has a fairly unusual mn (chosen for meaning) but how often is it out for everyone to see? Honestly, hardly ever! Of course many people know it but its not on the sign in board at school or what not. People hated it when I was pregnant and we told what it would be (except my phish head friends, lol) but now? It’s not like we say it all the time even in everyday conversation. That is what I love about the middle name spot! So anyways, [name]Leo[/name] [name]Maximus[/name] doesn’t personally sound over the top to me. And I also agree with the pp who said so many people [name]IRL[/name] are personally into such “safe” names for their kids that sure they [name]May[/name] wrinkle their noses at that name as they hand a juicebox to their own sons named [name]Aiden[/name] [name]Michael[/name] and [name]David[/name] [name]James[/name]. Now I personally like my first and middles to somewhat balance each other out…not having 2 unusual names, but that is just me.
I think a name is too over the top only if you think it’s too over the top
I really like [name]Leo[/name] [name]Maximus[/name]… It sounds super strong and masculine. I [name]LOVE[/name] [name]Leo[/name] [name]Xavier[/name]!
I think [name]Leo[/name] is lovely & not [name]OTT[/name] in itself.
I would choose whatever middle you like. [name]How[/name] many of your friends, colleagues etc do you know their mns? It is generally a private thing, so I think it’s fine to be a little theatrical/indulgent/whimsical.