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According to the FIR II IP I can't implement Butterworth Low Pass filter- only Decimation (and Interpolation and Decimation half-band…not Butterworth). Am I right?
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No. The FIR core can implement any kind of single- and multi-rate FIR filters, including decimation and interpolation filters. Butterworth filters are originally defined by polynomials (corresponding to an IIR topology). It's however possible to implement it as an FIR approximation. Filter coefficients can be calculated e.g. by Matlab or Nuhertz Filter Solutions tool.
You should be aware of a relation between fs/fc ratio and required number of FIR taps. A ratio of 50 k can be only effectively implemented as a multi-rate filter, the number of taps of a single rate is also in the fs/fc order of magnitude. In a usual IIR implementation, large fs/fc corresponds to a large extra bitwidth (log2 fs/fc) of the internal registers which is doubled for a second order biquad building block. This means, implementing a high order IIR filter with large fs/fc ratio is rather demanding. To reduce the internal bitwidth in higher order filters, it's possible to use an alternative topology similar to lattice wave digital filters.
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