The best performing ads are not necessarily the highest paying ads. What?? It's true Very few people understand this so let me use an example. Would you prefer to get an ad that pays you $1 per click but only gets 10 clicks a day or would you rather have an ad that pays you $0.40 per click but gets 100 clicks a day? PM if your need Help and Info on Adsense tips
yes you can use single ad on one page of "180x150". Although you will get low CTR but click worth or eCPM will be high.