οι αν βλαπτομενοι μη φυλαττωνται τους πολεμιους, υπ εκεινων μη αρχθωμεν
The best I can make of this is "Those who being harmed do not take precautions against [their] enemies, let us not be ruled by them." "Let us not be ruled by those who do not take precautions against their enemies when they are harmed."
I think this is the meaning of the middle of φυλάττω as used here (from Liddell Scott Jones; note: φυλάττω is the usual Attic form; φυλάσσω is generally Ionic but used by some Attic writers such as Thucydides):
II. φυλάσσεσθαί τι or τινα to beware of, be on one's guard against, avoid a thing or person, Sapph.27, etc.: “ταῦτα” Hdt.1.108, 7.130, cf. Ar.Ra.4; τινας A.Pr.715,804; “τοὺς Ἀτρείδας εἰσορῶν φυλάξομαι” S.Ph.455; “τέττιξ ποιμένας . . πεφυλαγμένος” Theoc.16.95.
μη modifies φυλαττωνται.
Does oi have an indefinite force like "whoever",
Yes. αν, μη and the subjunctive φυλαττωνται indicate that this is an indefinite relative clause. Here I would translate "those who".
There are two "quasi-conditional" constructions here that need to be distinguished: (1) the indefinite relative clause οι . . . φυλαττωνται, which is similar to a present general condition, and (2) the circumstantial participle βλαπτομενοι--within the indefinite relative clause--which can be viewed as conditional or temporal: if
they are harmed. In other words, there is a quasi-condition within a quasi-condition.
Does the tense of indicative in the apodosis then govern the tense of the phrase
Here, the verb in the "apodosis" of the condition represented by the participle βλαπτομενοι, i.e., φυλαττωνται, is subjunctive because this is an indefinite relative clause. Likewise, the verb in the "apodosis" of the indefinite relative clause, i.e., αρχθωμεν, is "hortatory" subjunctive (aorist). (Note: only the indicative of the indicative and aorist express tense; in other moods the opposition indicative/aorist is one of verbal aspect.)
But in answer to your question, the tense and mood of the verb in the "apodosis" of a condition in which the "protasis" is expressed by a circumstantial participle or an indefinite relative clause simply follows the usual rules for Greek conditions, depending on the type of condition that's involved.
As Daivid noted, this sort of question would probably be better submitted in the "Learning Greek" forum: it could be overlooked in this one. Hope this helps.