Welcome to the Dojo Slashuur-san!
Many members have given you great info thus far, so there is little I can add, other than this....
How much of the regular Usagi stories have you read so far? Have you read Usagai from the beginning? If not, maybe you want to start there...
If you have the comics, trade paper backs or hardcovers available to you, start from the beginning and work your way to the present. Read them in chronological publication order, and read the stand-alone titles according to when they were published in relation to the comics. That way everything can be read just as it would have when originally published. I know, that's a lot of reading, but probably the safest way the entire series and get the 'whole picture'.
Basically you can read Space Usagi at any point in time, since it is so distant in the future.
I'm not 100% sure about Yokai, but it appears to have been published in November 2009, which could fit it between:
Sep/2009 - issue #123 - The Death of Lord Hikiji and Nov/2009 - issue #124 - A Town Called Hell part 1
OR
Dec/2009 - issue #125 - A Town Called Hell part 2 and Feb/2010 - issue #126 - Nukekubi
(maybe Steve has a better idea as to exactly when & where).
Save Senso and read it between issue #144 (Shoyu part 2) and issue #145 (The Thief and the Kunoichi part 1), since that's when it was published.
I hope this helps, but no matter how you read them, I'm sure you'll enjoy them!
Now I think I should sit down and start over from the beginning myself...
p.s. - see Steve's Usagi Yojimbo Bibliography here:
http://www.usagiyojimbo.com/forum3/view ... f=2&t=7522