Q1: When this Digimon has 12000 DP or more and my opponent has 2 or more Digimon with 6000 DP or less, can I use this card's When Attacking effect to delete just 1 of my opponent's Digimon with 6000 DP or less?
No, you can't. This effect reads "delete," therefore you must choose and delete the specified number of targets whenever possible.
Lv.6 Siriusmon P-088 P Mega | Vaccine | Light Dragon [When Digivolving] By placing a Digimon with [Gammamon] in its name under this Digimon as its bottom digivolution card, this card gets +2000 DP for the turn. [When Attacking] Delete 1 of your opponent's Digimon with 6000 DP or less. If this Digimon has 12000 DP or more, delete 2 of your opponent's Digimon with 6000 DP or less instead.
Lv.6 Siriusmon P-088 P Mega | Vaccine | Light Dragon [When Digivolving] By placing a Digimon with [Gammamon] in its name under this Digimon as its bottom digivolution card, this card gets +2000 DP for the turn. [When Attacking] Delete 1 of your opponent's Digimon with 6000 DP or less. If this Digimon has 12000 DP or more, delete 2 of your opponent's Digimon with 6000 DP or less instead.
Lv.6 Siriusmon P-088 P Mega | Vaccine | Light Dragon [When Digivolving] By placing 1 card with [Gammamon] in its name from your hand as this Digimon's bottom digivolution card, this Digimon gets +2000 DP for the turn. [When Attacking] Delete 1 of your opponent's Digimon with 6000 DP or less. If this Digimon has 12000 DP or more, Delete 2 such Digimon instead.
Lv.6 シリウスモン P-088 P 究極体 | ワクチン種 | 光竜型 【進化時】自分の手札から、名称に「ガンマモン」を含むカード1枚をこのデジモンの進化元の下に置くことで、このターンの間、このデジモンをDP+2000。 【アタック時】DP6000以下の相手のデジモン1体を消滅させる。このデジモンのDPが12000以上なら、代わりにDP6000以下の相手のデジモン2体を消滅させる。