Of what I have been able to find, none of the surviving ancient source material for these myths tells us when the suit for Helen at Sparta occurred in relation to the Epigoni's battle on Lake Glisas, nor when Diomedes got married to Aegialeia [Aegialia] in relation to either of these aforementioned events. And Wikipedia (per the article you are citing) supplies no sources for the claim that Diomedes was wed after fighting at Thebes.
The best we would have available in the absence of such mentions is guesswork. I highly doubt the Wikipedia article is intentionally lining up a chronology between Diomedes' wedding on the one hand and the Epigoni's sacking of Thebes on the other hand. To me it reads like it's simply explaining the reasons for Diomedes' obligation to participate in the Trojan War rather than indicating that all of that paragraph's events necessarily take place after those narrated in the preceding one.
Notice that the last sentence before the one you've quoted about Diomedes becoming a suitor of Helen is discussing the death of Diomedes' kinsman Thersites during the Trojan War, an event which undoubtedly takes place after the suit of Helen. Based on this it seems quite reasonable to me to understand the statement about Diomedes becoming a suitor as simply mentioning a prior event in his life which led to his involvement in the Trojan War.
None of the surviving ancient mythography to which I refer, above, precludes the interpretation that:
- The Epigoni fought at Thebes and conquered the city.
- Shortly after that the famous Oath of Tyndareus was exacted from
the suitors of Helen.
- And then, as we know, only one of the suitors, namely Menelaus,
won the prospective bride's hand in marriage, and everyone else
went off to seek wives elsewhere. Among these unsuccessful suitors was Diomedes, who then, only at this point, went on to marry Aegialeia.
In The Greek Myths (Penguin Books, 1960), Robert Graves describes (on p. 630) the suit of Helen taking place when Diomedes had arrived at Sparta "fresh from his victory at Thebes". None of the ancient sources cited by Graves specifies such a sequence of events, so I suspect that Graves is simply deducing this based on certain variables, such as, perhaps, how young Diomedes is supposed to have been during the war at Thebes (around fourteen or fifteen years old), making it fairly implausible that he would have been suing for marriage earlier than that.
According to Carlos Parada's website the Greek Mythology Link (GML), there were indeed no commoners among Helen suitors. However, this does not mean that every single one of the suitors was necessarily the ruler of the domain from which he hailed. Several of them, as you say, were merely princes, and not especially the heirs-apparent to the thrones of their realms.
- Clytius was not the ruler of Oechalia at the time, his father Eurytus was.
- Telamon was still king of Salamis when his sons Ajax and Teucer each
went to Sparta to stake their claim upon Helen. Telamon continued to
rule even after the end of the Trojan War.
- Among the suitors there were three Cretans: Idomeneus, Meriones and
Lycomedes. Only the first of these was the king of the Crete, and
Meriones was his underling.
- Patroclus is listed among the suitors, and he may even have been an
exile (albeit one of royal lineage) at the time of the suit.
Apart from Diomedes, three other Epigoni are also listed among Helen's suitors, namely Sthenelus son of Capaneus, and the brothers Amphilochus and Alcmaeon. All three were Diomedes' relatives from Argos; and Amphilochus and Sthenelus are known to have become royal rulers in Argos alongside Diomedes only after the battle of Lake Glisas.
Alcmaeon was never such a ruler anywhere, it seems, but he is known to have been married. Sthenelus presumably was also married, as he had a son, Cometes, who is notorious for having cuckolded Diomedes during the Trojan War.
The stories of Sthenelus, Amphilochus and Alcmaeon as Epigoni suitors of Helen seem to be parallel to that of Diomedes: they were fairly young men (maybe even just boys) when they fought at Thebes; they won that war; went to Sparta each in a bid to become Helen's husband and failed at this; and then they found other brides elsewhere.