I recently started work at an English teaching company in Nagoya. There are a bunch of Japanese staff and they all speak English to some level. To most of them I speak in English and some in Japanese. I use keigo given I am new and they tend to use keigo as well I think due to the fact that I am a foreigner and they want to be nice/respectful.

This is all fine, but there are also two other foreigners there who have good Japanese (one is american and one is french/canadian). The American is actually my boss. In English I generally use casual and friendly English as they are about my age and nice guys. We joke around and have a good time. I don't feel any vibe of they are senior and I am new, except perhaps slightly with the American because he is such an experienced teacher (this is more my awe coming out of his awesome teaching skills than him being overbearing or anything).

Now, I have spoken Japanese with them a little bit, but I am not so sure what to use. I realise there is no real set answer here and since we are outside the culture we can just decide to use what we want. But anyway, do I use Japanese of the same tone as my English, which is to say friendly and casual? Or do I assume the Japanese mode of respect? As to do otherwise may not be speaking above my station in English, but it is in Japanese.

It doesn't really matter much, either way I'm sure they won't be offended, but I'm interested to see what you all think. I am using keigo as it is a bit of a gray area and it feels more Japanese like to me which is the language I am trying to learn/speak after all.

As another example. If you meet another foreigner in a Japanese speaking environment, do you use keigo or not? In English I would treat them like a friend more or less as I greatly value honest and open communication. In Japanese I would use keigo to be respectful.

I think that in an "office situation" one should take assume the "japananese mode" and use the style that is appropriate in the given social hierarcy. It might not be a big deal for a couple of foreigners but I would imagine for japanese bystanders it to be confusing if people speaking Japanese fluently ignore the expected norms. It might even give a wrong kind of message as Japanese is a highly context oriented language.
On the other hand, outside of work nothing matters in a sense..

I don't think that one should speak a language differently to foreigners than to native speakers as a language is a consistent set of rules of how to communicate. Keeping it consistent without letting anything, that's normal in some other language, affect it.
When speaking Japanese, speak the japanese way. When speaking English, speak the english/american/whatever way.
Helps to keep it simple(ish).

Just my 2c.

I would just use English, I mean why on earth wouldn't you???

The only time I can imagine this question coming up is if it was a group conversation and you wanted the Japanese people to understand your conversation. In that case I'd possibly be slightly more on the polite side.

I can't imagine another situation to when I would be using Japanese instead of English, but I'd probably be speaking fairly casually...

...maybe because he just wants to practice, and its fun to speak in Japanese, doesn't matter if they are foreign or not.

My shacho of the company I worked for, who was actually Japanese-American (and raised in America) but had lived in Japan for awhile and spoke fluent Japanese and English, made a good point about this. Basically, he said he had two sets of values, Japanese and American/English. When he spoke Japanese, you had to play by Japanese rules: keigo, etc. But when he spoke English, he was basically in American-mode, and you could talk to him in a rather friendly manner.

I myself feel the same way-- if someone is below me on the ladder for whatever reason, if we're speaking Japanese, lack of keigo/etc grates on me. But in English, there's much more leeway there, and I cut them more slack.

It's like a quote I heard one time, that I'll butcher now: when you learn a second language, you get another soul. In other words, I've developed another set of rules/sensibilities/etc for Japanese, so I adjust accordingly depending on the language I'm using.

I do actually want to speak Japanese with them for practice reasons. However, I need the practice more than them, my boss in particular (he has passed JLPT level 1), so I don't feel like trying to impose this (mainly for me) somewhat stilted form of communication on them.

This issue comes up when there is a conversation with the native staff in Japanese going on. It is weird and I think rude if I to switch into English to ask a question to the foreigners, since they other staff may not understand. Plus the conversation may as well be kept in the same language.

Outside of this in Japanese speaking environments I will use Japanese with foreigners. It's more fun that way

I think it is true you develop another soul in a new language. I will approach foreigners with keigo, but will switch to tameguchi quicker I think than if they were Japanese.