Feature selection in R too large dataset
Technically, it will be alright to use a sample of the data. One important assumption is that most of the features don't have outlier. In such case your sampling might miss out on those.

But, I don't see a necessity of sampling with the size of data that you have given. 150mb is not large data at all, given that you have 8 GB ram at disposal. Rather than sampling the data, first check if there is alternate way of selecting the features. Since there are only 18 columns, look at the summaries for each columns, how do the features correlate with your outcome variable, which variables add very little information. If you have not done more exploratory analysis of the 18 variables, I'll suggest you do that first. 

Secondly, why not build your random forest directly with 18 features? If you had, say, 180 features, it would have made sense to do automated feature selection first using Boruta etc. If there are features that do not add information, their variable importance in forest model will be low. You can drop them in subsequent analysis