The word order of clitics is really rather tricky, but your intuition in this sense was perfect!!

Unfortunately, apart from the word order, your sentences don't work at all
Keep in mind that "dať sa" is always an impersonal construction - it means "one can do X" or "X can be done", without mentioning
who would be doing X.
With verbs that take accusative objects, the object becomes the nominative subject:
Táto kniha sa dá prečítať za dva dni. = One can read this book in two days. This book can be read in two days.
Toto víno sa nedá piť. = It's impossible to drink this wine. (it's so bad)
Ten stôl sa tam nedá dať. = It's impossible to put that table there. (it doesn't fit)
With intransitive verbs, you get an "empty", impersonal subject:
Tu sa dá prejsť na druhú stranu cesty. (One can cross to the other side of the road here.)
Tebe sa nedá vyhovieť. (There's no way to please you.)
With verbs that already have
sa, one of the
sa is dropped:
Na tom sa dá len zasmiať. (dá sa + zasmiať sa)
Tu sa dá dobre najesť. (
dá sa + najesť sa)
with the consequence that the meaning can be ambiguous:
To sa dá ľahko naučiť. (dá sa + naučiť sa or
dá sa + naučiť) That can be easily learnt (or:
taught)
.
As for verbs that have the dative reflexive
si, my feeling is that it doesn't work very well:
? Tu sa dá dobre oddýchnuť. (doesn't sound very good)
?? Tu sa dá dobre si oddýchnuť. (sounds even worse)
Človek si tu môže dobre oddýchnuť. (this is what I would probably say)
This is a good place to rest.
But I haven't consulted any grammar books on this.
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In the barber situation, "
Čím by sa vám dalo poslúžiť?" (How could you be helped? How could one help you?)
would be grammatically well formed, but not idiomatic at all - perhaps because the impersoanl expression would imply that the speaker is not willing to help personally.
[minor edit - typo]