I read tasmaatiha = tasmaa + iti + iha; tasmaa = because of that (ablative
of cause); iti = what was said; iha = here. So, putting it all together:
"Because of what was said here," i.e. because of the preceding seven
questions and their answers and explanations.
bhikkhave = monks (of course!)
eva.m sikkhitabba.m: I agree with your translation, "you should train
yourselves thus:"
Then, in the part in single quotes (i.e. what the training is to be), I take
the subject to be te kaaraa, masculine plural, meaning "those ways," i.e.
those ways of using the requisites which the Buddha has pointed out as being
correct.
yesa.m ... tesa.m, dative/genitive plurals (but why, I'm not sure!)
we use the requisites of robes, alms-bowl, chair, bed, and support and
medicine for sickness
bhavissanti = they will be (i.e., those ways will be)
great fruits and great merits (both plurals, to agree with the subject, te
kaaraa)
amhaaha.m = for us
ca = and (because now we have the second part of a large compound sentence)
aya.m = this (nominative singular, feminine)
pabbajjaa = going-forth (nominative singular, feminine -- the subject of the
second part of the compound sentence)
bhavisati = will be (singular verb for the second part of the compound
sentence)
not in vain, will have a result.
SO ...... PUTTING IT ALL TOGETHER ....... I get something like this:
"Because of what was said here, monks, you should train yourselves thus:
'These ways of using the requisites of robes, alms-bowl, chair, bed, and
support and medicine for sickness will be of great fruits and great merits
for us, and this going forth will not be in vain, will have a result."