With practice skills will be fine, the only ones which are still annoying are up then left which in some directions can turn out up down, so you'll do a pretty useless heel to heel instead of a scoop turn. For a start just whore Ronaldo chops, that is very easy, as well as roulettes.
I usually make sure all players except CBs are 3* skill, and usually I can recognise the players by their hair and stuff so know if they are 3* or 4*.
Btw if you want to skill you HAVE to do it in Ultimate Team, skilling is hopeless in other modes and never works. In Ultimate Team the gameplay is changed and one of the few differences is much smoother and tighter dribbling so skills are very useful there.
Defending against the AI is usually tough, I usually just do a lot of R2+X defending (assuming PS3, not sure what it is on other systems), and perhaps the odd standing tackle. Slide tackles are very useful too, especially when timed well from behind or when the opposition players are counter-attacking. When timed well they are maybe 90% certain to work and not 40% like you say. When you're new to defending, a good strategy is to only make tackles with your full-backs, midfield and attack, and then use jockey defending with your CBs. Means you are unlikely to make too many costly mistakes, but you still have a good chance of a turnover in possession.
Against a human opponent is a lot tougher, as you have to learn the typical habits of human players which are very different to the AI's habits. The above tactic should I guess still work but human players are often adjusted to people whoring jockey defending so can still score easily. Another tactic I use is most people try to whore pace online so I just make my defenders run back instead of using contain, as with contain they'll never catch up but just running back they have a chance. It's a learning process and with time you'll get it.
In answer to your question, yes that is a good idea, try to make sure you are very close to the player then if you are almost sure you can make a tackle do it, otherwise if they are clean through you may have to take a chance with a wild slide tackle at the edge of the box. If you have many defenders back who are able to pick up this attacker, then make the tackle as it gives you a chance to counter attack but won't be too risky overall.
Btw jockey and contain are the same thing, I used both words here for some reason
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