No P-Bars, but can already do Ring Dips +135 lbs.

I've been doing dips on rings for a while and have done them with +135 lbs.  Would it be alright to skip from negative p-bar dips to ring dips?

No. Just because you have been doing ring dips doesn't mean you can do all of the single bar dips variations and l-sit russian dips properly.

Ideally you need to test for mastery on both the PE and IM. Maybe you have a park nearby that has some parallel bars you could use?

Chances are you are strong enough to demonstrate mastery off the bat, so suck it up and do the test to be sure

If you find a struggle, that's low-hanging fruit to make your dipping stronger still by brushing up a weak link. If not, on you go with no issue.

I used a dip station for these -- not p-bars.  Most gyms will give you a short trial membership for free -- probably just long enough to demonstrate mastery on these elements.

Just use two chairs.

Yeah, I'm just going to use two stools.  Now that I'm on the Korean dips, etc. I'm going to put a dowel in the rings.  Even though it's more unstable than a p-bar, I think if I control the movement, it will be pretty much the same.

Washienko said:

Just use two chairs.

That works really well for me also, just choose two sturdy chairs.

What are you guys using for Russian dips? (I'm not at that stage yet, but I want to make sure that I can find a way to do these at home)

builders trestles may be good for you, adjustable height, can be stored easily if they have swivel legs. try a google search and have a look