The Etruscans in the Syracuse list may be in there due to Thucydides mentioning Etruscans fighting in support of Athens during the siege of Syracuse (Thucydides 7.53 and 7.57). It’s not clear whether they were mercenaries, or were fighting as allies against a common enemy. I don’t think you can rule out Etruscans serving abroad as mercenaries, but they don’t seem to be mentioned often in that context (compared to Cretan archers or similar where there’s a well established reputation for mercenary service). I have my doubts about whether Etruscans mercenaries would have been in large enough numbers to constitute units of their own. More likely would be individuals or small groups serving in mercenary phalanxes alongside expatriates from other places. That said, the historical record is patchy at best and mercenary units of Etruscans could have existed.