Playing a 2v2 game with IG + Tau and the following situations came up:

IG using Tau marker lights to remove cover and increase ballistic skill. Is this allowable, or are these abilities useable by Tau only?

Assisted Overwatch, wherein IG tanks were charged and 4 nearby Tau units fired overwatch. Again is this allowable, or does it only work if a Tau unit is charged?

Seems a little overpowered when other Codices state "units from codex X only"...