Yup, That's a little tight...
Hongyi, suggest that you turn the skimmer 180 around... with the skimmer returns and beckett housing facing the external instead..... unless you like the idea of being a contortionist whenever you need to do adjustments to your skimmer....
Regarding the tight spacing.... try removing the skimmer without the housing and skimmer head.... if you can do that easily without getting any aches.... should be fine.....